Stereotactic Radiosurgery in the Management of Brain Metastasis

Michael L. Smith, M.D.; John Y. K. Lee, M.D.

Disclosures

Neurosurg Focus. 2007;22(3) 

In This Article

Which SRS System is Best?

There are two fundamental types of SRS systems. The prototype radiosurgical system is the Gamma Knife (Elekta) which uses 201Co-60 sources semispherically arranged around a geometric center. The basic engineering design concepts of the Gamma Knife have not changed since its development in 1967; design changes have increased usability and efficiency. This modality relies on forward planning with the delivery of "shots" to the tumor. It relies on the stereotactic Leksell G frame for rigid skull fixation and accurate dose delivery. The nomenclature "stereotactic radiosurgery" was coined by Lars Leksell, the Swedish neurosurgeon who invented the current Leksell arc-centered frame and the Gamma Knife. The precision and accuracy of GKS remain the standards by which intracranial SRS is defined.

The second type of radiosurgical system is based on linear accelerators, or LINACs, which are standard radiation oncology tools. The radiation source is mounted on a robotic arm and moves around the patient. Such systems include the Cyberknife (Accuray), X-Knife (Radionics), Trilogy (Varian), and Novalis (BrainLab). Early LINAC machines did not have the sophisticated features seen on modern units such as multileaf collimators, reverse planning software, and image-guided capabilities with cone-beam computed tomography scanners. Early versions were imperfectly adapted for precise cranial anatomy, resulting in poor quality assurance and, consequently, poor clinical outcomes compared with GKS.[43,46] Modern LINACs have gained sophistication. Some units allow non-frame-based stereotaxis, using a molded face mask or similar device. This alternative may appeal to patients who wish to avoid cranial pins and is more amenable to hypofractionated treatment regimens. Additionally, extracranial targets (such as spinal lesions) may also be targeted.

Most of the radiosurgical literature does not distinguish between GKS and LINAC SRS. The efficacy and safety of the two modalities are likely similar with the modern systems, although there is clearly a higher central dose, and thus more dose inhomogeneity, with GKS. Depending on the situation, this may serve as either an advantage or disadvantage. Choosing a particular SRS system is often based on institutional, financial, and administrative factors.[52]

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